The invention relates to T-type undercut electrical contact structure and methodology for fabrication on a semiconductor substrate.
To achieve low contact resistance, particularly low gate resistance in the case of a FET, a T-type undercut contact structure is used, as shown in FIG. 1. In the case of high frequency FET's, e.g. operating at millimeter wavelengths, the schottky metal first layer thickness is in the range of 1,000 to 1,500 angstroms, and the gold second layer thickness is in the 3,000 to 10,000 angstrom range. In such configuration, the gold can easily migrate along the side walls of the schottky metal and into the active semiconductor layer, as shown by the arrows in FIG. 2, causing device degradation and ultimately device failure. The migration of gold can be initiated as well as enhanced by the applied electric field, and further enhanced by the close spacing, e.g. 1,500 angstroms, at the undercut separating the gold from the semiconductor surface. The reason for the undercut is to define submicron gate length, e.g. less than about 0.5 microns, while retaining the low resistance requirement needed to achieve high performance and capability of operating at millimeter wavelengths.
The present invention overcomes the above noted difficulties, while retaining the design features of the T-type undercut contact structure. In a first preferred embodiment, a diffusion barrier is provided at the base of the gold layer to prevent it from diffusing into the semiconductor active layer. In a further preferred embodiment, the gold layer is encapsulated on the bottom and sides by a diffusion barrier.